Characterization and boron adsorption of hydrothermally synthesised allophanes

Two hydrothermally synthesised allophanes (Si/Al = 0.5 and 1) were modified by heating at 400 °C. N-methyl-d-glucamine, a specific boron complexant, was grafted on the allophane material with Si/Al = 0.5. The surface and structural properties of the different materials were characterised by infrared spectroscopy and 29Si CP MAS NMR spectroscopy. The porous characteristics of the different allophanes (specific surface area, pore size distribution) were determined by nitrogen adsorption at 77 K using the BET equation and the BJH model. The adsorption of boron, i.e. boric acid/borate, by the modified allophanes was studied in aqueous solutions with boron contents of 1 to 100 mg L− 1. The pH-dependence of boron adsorption of all the sorbents showed a maximum at a pH value close to the pKa value of boric acid (pKa H3BO3/B(OH)4− = 9.26). The adsorption isotherms of boron by raw and modified allophanes were fitted with a double Langmuir equation, assuming the presence of two sets of adsorption sites. The grafting of N-methyl-d-glucamine did not improve the boron adsorption of the allophanes but the structural changes by heating at 400 °C increased the adsorption.